Changes in gene and chromosome copy number are widely observed in systems from cancer to drug resistance to genome evolution. Despite the importance of aneuploidy to many important phenomena related to human health, little work has been done to determine how cells adapt to such extreme changes in gene dosage. Prior work from my lab using the model eukaryote yeast has found that aneuploidy can be detrimental to cells at high growth rates, but can be beneficial to cells growing in challenging environments. In nutrient- limited growth in chemostat culture, specific segments of the genome are reproducibly found amplified and deleted. In some cases, the proximal cause is obvious, such as amplification of nutrient transporter genes, but in others, such as those affecting large genome segments, the driving force remains opaque. The chemostat allows precise control over selection and growth conditions, and, importantly, a complete frozen history of each population, making this system ideal to study the role of aneuploidy in adaptation to strong, narrow selection. I propose to leverage this system to accomplish the follow specific aims: Aim 1: Determine the suite of copy number changes present in experimentally evolved cultures. Using a series of previously performed evolution experiments, we will survey populations for copy number changes using array comparative genomic hybridization and next generation sequencing. Aim 2: Determine the fitness consequences of genome rearrangements. Rearrangements found at high frequency in Aim 1 will be reconstructed and tested for fitness in direct competition assays versus matched ancestral strains. Rearrangements will be cross-tested in multiple selective conditions to query specificity. Results will be compared to evolved strains carrying multiple mutations to determine how much of their fitness benefit is due to genome rearrangements. Aim 3. Dissect the fitness contributions of each gene on the aneuploid chromosomes. To test the contribution of each gene on an aneuploid segment, we will take advantage of strain collections consisting of every yeast gene present at dosages ranging from deletion of a single copy to amplification to many copies. By competing these strains against each other and measuring their abundance via barcode sequencing, we can determine the fitness effect associated with every gene simultaneously. These data will be integrated and compared to the fitness effects of strains from Aim 2. We will also compete aneuploid strains in which each gene is returned to wt copy number to determine which genes are necessary for fitness improvements. This combination of approaches will be the first genome-wide attempt to dissect the precise molecular causes of the fitness changes associated with aneuploidy.